Signal strength controlled directed retry in a mobile radiotelephone system

Abstract

In a mobile radiotelephone system, a base station, upon receiving a service request from a mobile station, ascertains whether the service request can be accepted according to the availability of voice channels in the base station, and, if the service request cannot be accepted, compares a measured signal strength of the service request with at least one threshold value. If the signal strength exceeds the threshold, the mobile station is directed to retry the service request on one or more of a first set of channels assigned to neighboring base stations. Otherwise, the mobile station is directed to retry the service request on one or more of at least a second set of channels assigned to neighboring base stations. Accordingly, directed retries nearer the interior of a cell may be restricted or eliminated, whereas directed retries toward the periphery of the cell may be freely allowed or less severely restricted. Since interference is greater from mobile stations transmitting nearer the interior of a cell on the frequency of an adjacent cell than from such mobile stations transmitting from the periphery of the cell, interference is therefore reduced.

What is claimed is: 
     
       1. A method of controlling communications with a mobile station in a radiotelephone network, comprising the steps of: receiving a service request from said mobile station at a base station;   measuring at said base station the signal strength of a signal from said mobile station during said service request;   ascertaining whether said service request can be accepted based on the availability of voice channels assigned to said base station and, if said service request cannot be accepted;   comparing said signal strength with at least one threshold value; and   directing said mobile station to retry said service request on one or more of a first set of channels if said signal strength exceeds said threshold value; and otherwise directing said mobile station to retry said service request on one or more of at least a second set of channels.   
     
     
       2. The method of claim 1 wherein the channels in said first and second sets are variable. 
     
     
       3. The method of claim 1 wherein said threshold value is variable. 
     
     
       4. The method of claim 1 wherein said signal strength is compared with a plurality of threshold values and said mobile station is directed to retry said service request on one or more channels of one of a number of sets of channels, said number of sets being one more than said plurality of threshold values. 
     
     
       5. The method of claim 1 wherein one or both of said first and second sets is a null set, in which case said mobile station is directed, before more than several seconds have elapsed, to retry said service request to said base station. 
     
     
       6. The method of claim 1, further comprising the step of: normalizing said measured signal strength to account for actual transmission power of said mobile station, said normalized measured signal strength being indicative of a distance of said mobile station from said base station. 
     
     
       7. The method of claim 1 wherein said measured signal strength is normalized after said measuring and before said comparing. 
     
     
       8. An apparatus for controlling communications with a mobile station in a radiotelephone network, comprising: means for receiving a service request from said mobile station at a base station;   means for measuring at said base station the signal strength of a signal from said mobile station during said service request;   means for ascertaining whether said service request can be accepted based on the availability of voice channels assigned to said base station;   means, responsive to a determination by said means for ascertaining that said service request cannot be accepted, for comparing said signal strength with at least one threshold value; and   means, responsive to said comparing means, for directing said mobile station to retry said service request on one or more of a first set of channels if said signal strength exceeds said threshold value, and otherwise, for directing said mobile station to retry said service request on one more of at least a second set of channels.   
     
     
       9. In a cellular mobile radiotelephone system having a plurality of cells and a plurality of mobile stations, a method of reducing interference in said system of a call made by a mobile station when setting up said call within a congested cell, comprising the steps of: obtaining an indication of said mobile station's location relative to a probability of interference with established calls by setting up said call in said congested cell;   freely allowing said mobile station to retry setting up said call in at least one neighboring cell when said mobile station is indicated as being located in an area of said congested cell where interference with established calls by the setting up of said call is likely to be small; and   restricting said mobile station from retrying setting up said call when said mobile station is indicated as being located in an area of said congested cell in which interference with established calls by the setting up of said call is likely to be greater.   
     
     
       10. The method of claim 9 wherein said obtaining step comprising the steps of: measuring, at a base station, said congested cell signal strength of a signal transmitted by said mobile station on a control channel of said base station; and   normalizing said measured signal strength to account for actual transmission power of said mobile station, said normalized measured signal strength being indicative of a distance of said mobile station from said base station.   
     
     
       11. The method of claim 10, wherein said obtaining step further comprises the step of comparing said normalized signal strength to a threshold value. 
     
     
       12. The method of claim 9, wherein said step of freely allowing said mobile station to retry said call comprises the step of sending a directed retry message to said mobile station listing a first set of candidate cells for retry of said call. 
     
     
       13. The method of claim 12, wherein said step of restricting retry by said mobile station of said call comprises the step of sending a directed retry message to said mobile station listing a second set of candidate cells different from said first set for retry of said call. 
     
     
       14. The method of claim 12, wherein said second set is a null set. 
     
     
       15. In a cellular mobile radiotelephone system having a plurality of cells and a plurality of mobile stations, an apparatus for reducing interference in said system of a call made by a mobile station when setting up said call within a congested cell, comprising: means for locating said mobile station relative to a probability of interference with established calls by setting up said call in said congested cell;   means for allowing said mobile station to retry setting up said call in at least one neighboring cell when said mobile station is located in an area of said congested cell where interference with established calls by the setting up of said call is likely to be small; and   means for restricting said mobile station from retrying setting up said call when said mobile station is located in an area of said congested cell where interference with established calls by the setting up of said call is likely to be greater.   
     
     
       16. The apparatus of claim 15, wherein said means for locating comprises: means for measuring, at a base station, said congested cell signal strength of a signal transmitted by said mobile station on a control channel of said base station; and   means for normalizing said measured signal strength to account for actual transmission power of said mobile station, said normalized measured signal strength being indicative of a distance of said mobile station from said base station.   
     
     
       17. The apparatus of claim 16, wherein said means for locating further comprises means for comparing said normalized signal strength to a threshold value. 
     
     
       18. The apparatus of claim 15, wherein said means for allowing said mobile station to retry said call comprises means for sending a directed retry message to said mobile station listing a first set of candidate cells for retry of said call and means for storing a list of said candidate cells in said first set. 
     
     
       19. The apparatus of claim 18, wherein said means for restricting retry by said mobile station of said call comprises means for sending a directed retry message to said mobile station listing a second set of candidate cells for retry of said call and means for storing a list of said candidate cells in said second set.